(Brown ef al., 1966) have all been shown to be increased as a result of increased

irradiation. Such compensation in the leafless pea canopy, however, may only be

significant at population densities in excess of 44.4 plants/m2, since 50% or more

of the light falling on canopies composed of lower plant populations fails to be

intercepted. The extent to which photoassimilate produced by the photosynthetic

structures below the first flowering node is incorporated into developing fruits is

unknown. It has been shown that the major photosynthetic contribution to the

fruits of spaced plants is derived, in leafless and leafed peas, from the tendrils or

leaflets subtending each developing pod (Harvey, 1974). It has been suggested

that the photosynthetic potential per unit area of the tendrils of the leafless mutant

may be higher than for the leaflets of the leafed phenotype (Harvey and Goodwin, 1978). If this proves correct, then tendrils subtending pods would be more

efficient and compensate to some extent for the poor light interception at the top

of the crop.

One consequence of more light penetrating the leafless canopy to soil level is

an increase in soil temperature compared with corresponding leafed canopies

(Table I). At 100 plantslm’ a soil temperature of 29.8”C in the leafless canopy

was reduced to 24.2”C in a corresponding leafed population. This temperature

difference increased to 8°C when the populations were reduced to 25 plants/m2.

The physiological consequences of this increased soil temperature are not

known. It is likely, however, that there will be a higher rate of evaporation from

the soil surface of the “leafless” crop, although this may be more than compensated for by a decrease in the rate of water utilization by leafless plants (Harvey,

1980). Although soil temperatures differed between the two phenotypes, air

temperature within the canopies were much more similar, and at 100 plants/m2

were not significantly different. Greater air movement in the leafless canopy may

compensate for the increased level of radiation penetrating the crop, although at

present there has not been any research on this aspect of leafless canopy structure.

The differences in light interception between leafed and leafless canopies

C. L. HEDLEY AND M. J . AMBROSE

232

Table I

Comparison of Temperatures (“C) through the Crop Canopies of JI 1194 (Leafed) and

JI 1198 (“Leafless”) at Two Planting Densities

Planting density (plantsh’)

Height

within

canopy (cm)

25

I00

JI 1194

JI 1198

JI 1194

JI 1198

25.6

25.2

25.4

24.9

24.9

26.4

24.6

25.6

26.2

27.2

27.5

28.7

24.2

29.8

27.5

35.5

~

30

20

10

1 cm below

soil level

correlate with the differences between the two phenotypes in total above-ground

biological yield per unit area (Fig. 4a). The biological yield of the leafed

canopies decreased marginally as the planting density was increased. The leafless

phenotype, however, had a biological yield per unit area at the lowest planting

density (16 plants/m2) that was approximately half that attained by the corresponding leafed canopy. At higher planting densities the biological yield per unit

area of the leafless phenotype increased progressively and at densities in excess

of 100 plants/m2 exceeded that attained by the leafed canopies at any of the

population densities.

The responses observed on a unit area basis are obviously a reflection of the

responses of individual plants within the population. As the space available to

individuals is increased (plant density decreased) so plants will take advantage of

increased resources and grow larger. There will therefore be a tendency for low

plant populations to compensate by each plant contributing more biological yield

to the total biological yield per unit area. This explanation, although true, is an

oversimplification of the effect of planting density on the individuals within the

population. The “average” plant response, as determined by dividing the response per unit area by the number of individuals, does not convey the differences between plants within each population, induced by interplant competition. The use of average plant values, while ignoring these complex plant-toplant interactions, does, however, indicate how genotypes (or phenotypes) are,

in general, responding to particular environments.

Over the range of planting densities, the average leafed plant (Fig. 4b) had the

capacity to compensate for a 25-fold increase in the available space [25 cm2/plant

(400 plants/m2)to 625 cm’/plant (16 plantdm’)] by a 30-fold increase in biological yield. The average leafless plant (Fig. 4b), however, could respond to a

similar increase in space by only a 12-fold increase in biological yield. The

biological yield per unit area of the leafless phenotype was therefore greatly

233

DESIGNING “LEAFLESS” PLANTS FOR DWED PEA CROP

reduced at low population densities. An alternative view is that there was a steep

reduction in the biological yield of average leafed plants as the space available

was reduced, whereas the effect on average leafless plants was less severe. The

result of this differential effect on average plants of the two phenotypes was for

the biological yield per unit area of the leafless phenotype to exceed that of the

leafed when the space available was less than 100 cm2/plant (density greater than

100 plants/m2).

Up to a density of approximately 100 plants/m2, the effect of planting density

on the economic yield per unit area (seed dry weight per square meter) of the two

phenotypes (Fig. 5a) was similar to the effect on biological yield (Fig. 4a). Seed

weights per unit area of the leafed phenotype decreased marginally between 16

and 100 plants/m2, whereas those of the leafless phenotype increased progressively up to this density. As with the biological yield, the economic yield per

unit area of the leafless phenotype exceeded that of the leafed at densities of

approximately 100 plants/m2 and greater. The economic yield per unit area of

both phenotypes, however, was significantly reduced between 100 and 400

plants/m2.

7-

5-

6

4 -

b

h

c

I

2

7 5

0,

3-

Y

c

Y

c

N

0

E

k4

2 2 -

\

0

3

1 -

2

l

”

7

”

6

“

5

1

4

A

A

( 16

25

3

2

A

44

1

A

0

A

100 400)

0-

7

6

5

4

3

2

A

A

A

( 16

25

44

1

A

loo4oo)

Square of Distance Between Plants ( c m 2 x

FIG.4.

0

A

Biological yield in grams per square meter (a) and grams per plant (b), of leafed

(0-0) and leafless ( 0 4 )phenotypes, over a range of planting densities. Each phenotype is

the mean response of three genotypes. Numbers in parentheses are the planting densities (plants per

square meter).

234

C. L. HEDLEY AND M. J . AMBROSE

4

3

N

I

b

2

N*

E

2

1

a

l

"

7

6

"

5

"

2222%

7

l

4

3

2

1

0

A

A

A

A

( 16

25

44

100 400)

A

6

A

(16

5

4

3

2

1

0

A

A

A

25

44

looroo)

Square of Distance Between Plants ( an2 x

lo-'

)

Economic yield in grams per square meter (a) and grams per plant (b), of leafed

phenotypes, over a range of planting densities. Each phenotype is

the mean response of three genotypes. Numbers in parentheses are the planting densities (plants per

square meter).

FIG.5.

(0-0) and leafless (0-0)

The effect of planting density on the seed yield of average plants (Fig. 5b) was

more extreme than the effect on biological yield (Fig.4b). This was most marked

for average leafed plants that had a 45-fold reduction in seed yield per plant when

distribution. As growth continues there will be an increasing proportion of small

plants and a decreasing proportion of large plants within the population. Donald

(1963) suggested that crowding accelerates this process and that it is due to

increased variability of relative growth rate in crowded communities.

An increase in the variation between plants occurs as competition becomes

intense and is reflected in an increase in the coefficients of variation (CVs). For

example, Stem (see Donald, 1963), using subterranean clover, found that the CV

for plant weight was similar at all densities for 90 days and then increased sharply

at the higher densities. Certain plant characters respond more to increased competition than others. It has been shown for Zea mays (Edmeades and Daynard,

1979) that the CV for plant dry weight and for ear components increases with

plant density, whereas the CV for plant height and leaf area per plant were little

affected. Hozumi et al. (1955), in one of the few experiments specifically

designed to study the effect of competition on individual plants, found that

yellow dent corn plants within a row oscillate for weight and shoot length

between negative and positive relative to the overall plant mean. It was apparent

that if a plant grew vigorously, its neighbors were suppressed, and if its growth

were retarded, the neighbors were favored in their growth.

At very high planting densities distributions for plant size become so skewed

that self-thinning occurs. Donald (1 963) demonstrated that self-thinning in

wheat occurs to a density greater than that giving the highest grain yield per unit

area. This suggests that within a dense population survival of individuals has

precedence over total seed production per unit area.

How variation between individual plants in the population relates to the yield

DESIGNING “LEAFLESS” PLANTS FOR DRIED PEA CROP

239

per unit area of the crop and to stability of crop yield is not clear. It is assumed

that populations where the yields of individuals are distributed normally are to be

preferred to those where the distribution is skewed and the CVs are high. In other

words, a crop where all of the individuals yield something is to be preferred to a

crop where a few individuals yield much while others yield little or nothing.

It is the degree of competition between plants at high planting density that

determines how much individuals differ from each other. The extent to which

individuals interact within a crop is dependent on the competitiveness of the

individuals, the planting density, and the environment. Since leafless plants must

be grown at high planting densities, we can only reduce the interaction between

individuals by selecting genotypes that are less competitive or more tolerant of

high population densities. This is a similar conclusion to that derived for wheat

by Donald (1968), who states that, “The individual plant within the community

will express its potential for yield most fully if it suffers minimum interference

from its neighbours. ” Neighboring plants should therefore be weak competitors,

and the ideotype itself must be of low competitive ability.

It is not, however, sufficient to define a crop plant solely by its tolerance of

other individuals at high planting density. An ideal crop plant should also have a

high efficiency for partitioning its assimilate into economic yield. Donald (1968)

has stated for wheat that, “The successful crop plant should be of low competitive ability relative to its mass and of high efficiency relative to its environmental

resources.” Therefore, in the following sections of this article we discuss those

characteristics of leafless peas that may be incorporated into the ideotype to make

it more tolerant to high planting density and maximize biological yield per unit

area. We then define, to the best of our knowledge and experience, characters

that will maximize the efficiency with which biological yield may be partitioned

into economic yield.

111. AlTAINING MAXIMUM BIOLOGICAL YIELD PER

UNIT AREA

A. IDENTIFYING PLANTSTHAT ARETOLERANT

OF INTERPLANT

COMPETITION

The main conclusion from the previous section was that the most suitable

leafless crop plant will be tolerant of its neighbors at high planting densities.

“Average” plant responses can be used to indicate those characteristics that will

be advantageous to a genotype in a competitive environment. Average plants of

strongly competitive genotypes will greatly increase their biological yield when

grown at low planting density. When such plants are grown at high planting

240

C. L. HEDLEY A N D M. J . AMBROSE

density, however, they will compete so vigorously that the yield per average

plant will be drastically reduced. The density response for an average strongly

competitive plant will therefore be extreme and the slope of this response will be

steep (Fig. 8a). Weak competitive genotypes with the same duration of growth as

strong competitors, will not take full advantage of the resources available at low

planting densities and average plants will therefore have lower biological yields

than strong competitors. When such genotypes are grown as dense populations,

their reduced aggressiveness in competition for resources will reduce the interaction between individuals, and the yield per average plant will be less affected and

may be higher than that of the strong competitor. The density response for an

average weakly competitive plant will therefore be less extreme and the slope of

this response will be shallow (Fig. 8a).

The effect of density on an average individual is determined from measurements made per unit area of the population. The population-density interactions for strong and weak competitors (Fig. Bb), however, will be the inverse of

those for the average individual within a population (Fig. 8a). Yields per unit

area from a monoculture composed of a genotype that is tolerant of high planting

density will therefore show a steep positive response to planting density (Fig.

8b), whereas a population composed of a genotype that is intolerant of competition will show a shallow or even negative response to increased density (Fig. 8b).

Therefore, in theory a genotype's ability to tolerate competition can be determined from a comparison of yields per unit area of populations grown at a low

(noncompetitive environment) and at a high (competitive environment) planting

density. This comparison can be made to test the relative effect of specific

genotype characteristics on tolerance to competition. Comparisons using strongly